Switching to VoIP

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Each device handles ALS in a proprietary way, though all ALS solutions apply the

distributed computing model to solve the remote survivability problem. Those that

are integrated with a central PBX make a local copy of the dial-plan so that local

users can still call one another when the link to HQ is broken. Those that have an

FXS port allow it to be used for emergency 911 calls regardless of the state of the cen-

tral PBX.

Some vendors have special nicknames for ALS. Cisco calls its ALS technology SRST

(Survivable Remote Site Telephony), and Avaya calls it LSP (Local Survivable Proces-

sor). Most ALS solutions include:

• A local cache of the dial-plan so that phones at the ALS-equipped remote site

can still call each other without the central PBX server.

• Instructions on how to handle calls to the private voice network from the remote

site during times when the IP path to HQ is down. These instructions might

mean diverting the calls over the PSTN and into the HQ site’s PSTN trunks.

• Parameters on how long to wait after an outage occurs before attempting to give

call control back to the central PBX at the HQ.

• Parameters that describe when and how often to replicate the local cache of the

dial-plan.

• The ability to route 911 calls to a locally connected POTS line ...